The invention relates to parachute flares, or flare candles, for illuminating projectiles, such as the conventional M485, 155mm illuminating projectile which is launched by a 155mm howitzer cannon toward a distant enemy area and initiated at a burst height of about 600 meters (M) to eject the parachute flare to burn during its parachute descent.
Normally, such flares contain a single conbustible formulation, resulting in a uniform burn rate and candlepower output. Since the illumination, in foot-candles, on a ground target varies inversely as the square of the distance from the light source, the ground illumination during a large part of the burn is insufficient for proper observation. The problem originates because of the necessity to maintain a relatively long burn time. Since the rate of fall or descent of the parachute and flare is nearly constant, at about 14 ft./sec., this requirement leads to higher burst heights, which require higher intensity (candlepower) flares. If the intensity is sufficient to provide adequate illumination during the first part of the burn, it is far higher than necessary during the last part of the burn, and hence inefficient.
Tests have shown that the flare for the original M485 illuminating projectile, which is a hollow metal or paper cylinder having a length of about 4.5 inches, open at end and closed at the other end and filled with a single flare composition of 52% Mg, 40% NaNO.sub.3 and 8% polyester resin binder, when ignited, burns at an intensity of about 3,500,000 C.P. for only about 45 seconds. With a descent rate of 14 ft./sec., the flare will descend about 630 feet in this time. If the flare shell is initiated at a height of 300 M (984 feet), the ground illumination at the start of the burn is ##EQU1##
Currently, particularly in metal case applications, the interior walls of the flare case or cylinder are coated, before filling, with a minimum prescribed thickness of a resin material, such as polyester of epoxy resin, which acts as an inhibitor/insulator, reducing the burn rate and increasing the burn time. This has also been done with flares using paper-cardboard cases. The effectiveness of the resin coating is a function of its thickness. When the flare case of the M485 illuminating projectile is provided with such a resin coating having a thickness of 0.03 inch before filling with the illuminating composition, the flare burns at an intensity of about 800,000 C.P. for about 120 second. If the burst height is 600 M (1968 feet) the ground illumination is about 0.2 foot candle at the start, and about 9.6 foot candles at the end of the burn. Although the burn time (120 seconds) is adequate and the initial illumination is equal to the normal requirement of at least 0.2 foot candle of ground illumination, the final illumination is much higher than necessary.
Recent efforts have been directed toward a variable burn rate flare. One method is to consolidate a candle (flare) with three different formulations. The first increment is an extremely rapid formulation delivering the greatest candle power output or intensity at the height of the burn (the greatest distance from the target); the second and third increments are formulations of decreasing burn rate for use in succession as the candle descends closer to the target. This method is very expensive to carry out and control since the three formulations must be integrated, and would be a production bottleneck in any automated system of manufacturing flares.
In accordance with the present invention, a variable burn rate/variable burn time flare is produced by coating only a predetermined portion of the inner case wall, at the closed end, with a thin layer, e.g. 0.03 inch thick, of a polyester or epoxy resin, leaving the remaining portion of the wall at the open end bare; and then filling the case with a single illuminating formulation. When the flare is ignited, at any desired height, the portion of the flare composition located within the bare open portion of the case wall burns for a first period of time at a rate that is about midway between the burn rates for a completely bare flare case and a fully coated flare case, and then burns for a second period of time at about the same rate as a fully coated flare. Preferably, the flare is designed and the burst height is chosen so that the total burn time is equal or nearly equal to the time of descent of the parachute flare. The burn rates and burn times are adjusted by adjusting the fraction of the case wall that is coated. The outer edge of the coating may be tapered in thickness to produce a gradual transition between the coated and bare portions. Several examples are given, with different fractional coatings and different burst heights.